Substrates of organic resins such as polycarbonate resins, acrylic resins and PET resins are now utilized as building materials, structural materials and optical articles (e.g., eyeglass lenses) by taking advantage of their transparency and impact resistance as well as light weight and low cost. For example, as the window glass of automobiles, transparent organic resins substitute for prior art inorganic glass to achieve great contributions to weight reduction and fuel consumption savings. However, since these organic resin substrates are susceptible to surface flaw which detracts from transparency, they are often coated with high hardness coating materials.
It is known from the past that the preferred coating materials are hardcoat compositions comprising a silica dispersion and a heat curable siloxane resin. This is because siloxane resins are generally resistant to degradation by sunlight and ultraviolet radiation and thus quite suitable for surface protection over a long period of time.
However, these hardcoat compositions have several drawbacks. For example, the adhesion between a hardcoat layer and a resin substrate is insufficient. One known approach for improving the adhesion is to form a primer layer on the resin substrate. Since the hardcoat layer itself is composed mainly of siloxane resin and silica and lacks an ability to shield ultraviolet radiation, the resin substrate, the primer and the interface therebetween can be degraded or discolored by ultraviolet radiation. For prohibiting this phenomenon, it was proposed to add a UV absorber to the primer layer.
The hardcoat layer has a high hardness and mar resistance, but the high hardness, in turn, gives rise to a potential problem that the hardcoat layer cracks. A solution to this problem was proposed in JP-A 6-73329 which discloses a hardcoat composition comprising a silica-dispersed solution of a partially hydrolyzed oligomer of hydrolyzable organosilane, and a silanol-containing polyorganosiloxane. However, a hardcoat film of this composition formed on a plastic substrate is less resistant to marring because the hydrolyzable organosilane is restricted to a partial hydrolyzate.